| Dolomite have great value for oil and gas exploration,Scholars have researched on the origin of dolomite for many years and established different genetic models.However,there are still many puzzles about the origin of dolomite so far.There is dolomite with potential high-quality reservoir in Tongzi formation of Lower Ordovician in Sichuan Basin.Based on this,this paper takes the dolomite of Tongzi formation of Lower Ordovician in South Sichuan as the object,the petrological and geochemical characteristics and genesis of dolomite in the lower Ordovician Tongzi formation in southern Sichuan are systematically studied by means of whole rock X-ray diffraction,cathodoluminescence,porosity and permeability test,common and trace elements,carbon and oxygen isotopes,strontium isotopes and rare earth elements,and obtains the following understandings:The dolomite of Tongzi formation in South Sichuan is mainly developed in the middle part of the formation,and the sedimentary structure is mainly horizontal bedding.Dolomite of Tongzi formation is widely developed in the basin,especially in South Sichuan.According to the crystal size,the dolomite in the study area can be divided into mud-micro dolomite and powder-fine dolomite,and mud-micro dolomite is the most developed one.The physical properties of dolomite are generally better than that of limestone.The original structure is preserved The physical properties of mud-micro dolomite with texture are better than that of powder-fine dolomite with residual texture.Mud-micro dolomite crystal is small,crystal shape is difficult to identify,particle content is less,original structure is well preserved,quartz particles can be seen.The order value of dolomite is low,and it is dark red under cathodoluminescence.Mg/Ca ratio,Na,Sr,Fe,Mn,Si O2,Al2O3content are higher.The carbon and oxygen isotopic composition of dolomite is close to that of seawater.The characteristics of rare earth elements are similar to those of micrite limestone in the same period.The average value of 87Sr/86Sr is close to the value of 87Sr/86Sr of micrite limestone in the same period.The results show that the mud-micro dolomite was formed quasi syngenetic.At the same time,it was mixed by terrigenous clastic materials in the process of sedimentary diagenesis.The powder-fine dolomite is composed of the fine-grained dolomite and the fine-grained dolomite.Most of the crystals are in the shape of half automorphism or curved surface,and they are nearly inlaid cementation.The grain structure is generally developed,the residual grain structure is more common,and the recrystallization is significant.Under the cathodoluminescence,the core of dolomite crystal is dark red,and the dolomite crystal with bright edge and band structure can be seen.The contents of Na,Fe,Mn and Sr are relatively high,and the isotopic compositions of carbon and oxygen are close to those of dolomite of seawater origin in the same period,while others show obvious lowerδ18O value,and the characteristics of rare earth elements are similar to those of micrite limestone in the same period.The distribution of 87Sr/86Sr in powder-fine dolomite is wide,and most of them are close to the 87Sr/86Sr values of micrite limestone in the same period.The results show that the powder-fine dolomite is the product of the early stage of quasi syngenetic dolomitization and the late stage of diagenesis superimposed and buried dolomitization. |
